One type of drug delivery device known in the art is an autoinjector which contains a medical, therapeutic, diagnostic, pharmaceutical or cosmetic compound (drug) before it is administered, and which is used to administer the compound through the skin of the patient via a hollow needle. Autoinjectors may be used by the patient themselves or by a different user, and are also used to administer drugs to animals.
Autoinjectors are typically used because they reduce the amount of training and effort needed by a user compared with that needed for a syringe, by automating either or both processes of inserting the needle into the patient and expelling the drug through the needle. They can also reduce the fear of injection by hiding the needle from the patient.
Some autoinjectors use a single spring to provide the motive power to both insert the needle into the patient and deliver the drug. Examples of this approach include the EpiPen autoinjector from Meridian and the Humira autoinjector from Abbot.
Where an autoinjector includes only one spring to provide the force to drive both functions, the force that the spring provides for one of the functions may be higher than needed, to enable the spring to provide sufficient force for the other function. Advantageously the two functions happen one after another rather than simultaneously in order that the drug is delivered only after the needle is correctly positioned. Because the force provided by a spring typically reduces as the spring delivers energy, the spring inevitably provides a higher force for driving the first function, i.e. needle insertion, than for the following function i.e. drug delivery, whether or not this is desirable. The strength of the spring is determined by the requirement for the spring to be able to provide sufficient force and energy at every point during the drug delivery process. This often means that much higher force than is needed or desirable is provided during the needle insertion phase.
However some autoinjectors use two separate springs within their operating mechanisms to provide the motive power to insert the needle and deliver the drug. Examples of this approach are described in U.S. Pat. Nos. 4,642,099 and 7,749,195.
Where an autoinjector includes two springs, the force provided by each spring can be tailored to better suit the requirements of each function. The overall maximum stored spring force required in the autoinjector mechanism can be significantly reduced, because each spring on its own no longer needs to provide sufficient energy to drive both functions.
The use of two springs in this way typically requires an interlock mechanism to coordinate the sequence of the two functions so that significant drug is not expelled through the needle before the needle is inserted correctly into the patient. The interlock mechanism typically adds size and complexity to the autoinjector.
It is an object of the present invention to provide an improved mechanism to control the sequence of release of two drive members, such as springs, within an autoinjector, which allows the size and complexity of the autoinjector to be kept to a minimum.